The present invention relates to a novel porous hydroxyapatite material suitable for use as an artificial bone substitute or, more particularly, to a porous hydroxyapatite material usable as a granular filling in a cavity or a lost portion of bones and also as a sintered body for prosthesis of bones in the fields of dentistry, stomatoplasty, orthopedics and the like.
One of the problems in the dental treatment is absorption of the alveolar bone after tooth extraction to cause instability in the fixation of an artificial tooth necessitating a filling material. In the treatment of stomatoplasty and orthopedics, it is a common practice to undertake prosthesis of bone lost in a traffic accident or as a result of bone tumor with a material for bone substitute. Although autografting of bone, i.e. implantation of a piece of bone taken from a part of a patient's body to the lost portion or cavity in the bone of the same patient, is a preferable way from the physiological standpoint, a very serious physical and psychological load is unavoidable on the patient in the autografting of bone because the bone piece used for autografting must be taken from an unaffected bone tissue of the patient himself. Needless to say, no bone piece for autografting having a sufficiently large volume can be obtained to cover a large lost portion of the bone. Accordingly, intensive investigations have been and are being undertaken to develop an artificial material for bone substitute as a filling of a bone cavity or as a prosthetic member of a bone. Such an artificial bone substitute material must satisfy several requirements including, for example, a high degree of safety or absence of toxicity, sufficient mechanical strength, good affinity to the living body tissue in which the material is embedded or implanted to form a firm bond therebetween, and others. Further, it is sometimes desirable that the implanted artificial material is spontaneously absorbed and disappears in the lapse of time to be replaced with a neogenetic bone tissue. In this regard, prosthetic members made of a metal such as titanium, stainless steel, aluminum and the like are not always satisfactory due to the lack of metabolizability despite their high mechanical strengths. When a prosthetic member made of these unmetabolizable materials is used, it must be removed by operation after complete cure or must be left remaining in the patient's body lastingly though with an apprehension of some adverse influences thereby as a foreign body.
A class of promising artificial bone substitute materials recently developed in view of these requirements include tricalcium phosphate, hydroxyapatite, specific calcium phosphate of an apatite-like crystalline structure and the like in a sintered form and extensive investigations are now under way to develop artificial bones, artificial articulations, artificial dental roots and the like using these materials.
It is a desirable condition that the artificial bones, artificial dental roots and the like substitute bodies have a porous structure in order to facilitate formation of a firm bond between the living body tissue and the substitute body implanted therein by the proliferating growth of the living body tissue into the pores of the porous substitute body. In this regard, a proposal has been made in Japanese Patent Kokai Nos. 56-149389 and 57-7856 for the use of a porous sintered body of calcium phosphate in which the pores have diameters in the range of about 0.03 to 1.2 mm. This porous sintered body of calcium phosphate is, however, defective as an artificial bone substitute due to the brittleness and insufficient mechanical strength. The material is also not quite satisfactory in respect of the absorptivity in the living body and replaceability with a neogenetic bone tissue due to the dense texture of the base material.
In respect of a granular filling material used in a cavity of bones, a proposal is made in Japanese Patent Kokai No. 56-54841 of a powdery product of calcium phosphate having an apatite-like crystalline structure with a crystallite diameter in the range from 5 nm to 10 .mu.m which is used for filling a cavity or a lost portion of a bone in the form of a slurry with addition of a physiological saline solution. A problem in this powdery material is that, because the particles thereof are so finely divided, particles sometimes adhere to the traumatic surface of the skin to cause difficulties in suture of the trauma.